Poliomyelitis is a disease in the human central nervous system caused by a polio virus. The polio virus is divided into 3 serotypes i.e. types 1, 2 and 3, and it is considered that there are a variety of naturally occurring strains having weak (attenuated virus strains) to strong (virulent virus strains) levels of neurovirulence to which the primates only are sensitive.
Natural infection and prevalence of polio have occurred exclusively in the human being since ancient times as an infectious disease. A large number of humans still become infected with polio every year in developing countries. Hence, the eradication of polio in the near future is a task of the human being, and it is expected that a live polio vaccine can be the most effective weapon to solve this problem.
Although an attenuated polio virus has been produced and used as attenuated oral polio vaccine, the attenuated polio viruses may be dangerous for the possible reversion of pathogenicity (paralysis-based neurovirulence) in persons administered or in contact with it. Hence, there is a need for a safe and effective polio vaccine which is free of such pathogenicity. To produce such polio vaccine, consistency tests, particularly tests for neurovirulence of vaccine virus in monkeys, have been indispensable test.
Neurovirulence tests using monkeys, carried out worldwide at present, involve inoculating a predetermined amount of polio virus into the spinal cord of a monkey where the frequency of occurrence of paralysis and the histopathological changes in the central nervous system are clinically observed and numerically expressed in terms of "lesion score". In these prevailing tests for neurovirulence of attenuated polio viruses, a large number of monkeys such as cynomolgus monkeys are used.
However, these neurovirulence tests using monkeys suffer from the following disadvantages: (1) the price of the monkey is too high compared with conventional experimental animals; (2) a person handling the monkey is in the danger of infection with a pathogenic virus from the monkey; (3) the supply of the monkey declines for the protection of wild animals; and (4) wild monkeys are not homogeneous in genetic characteristics, resulting in data fluctuations even with the same sample (scattering results are inevitable).
In spite of these disadvantages, mice and rats cannot be used because such animals other than primates are not sensitive to the polio virus as described above.